1. Field of the Invention
This invention relates to methods for inspecting phase-shifting masks for photolithography, and more particularly to phase-error enhancement by inspection.
2. Description of Related Art
The phase-shifting mask technique has been shown to improve the imaging performance of optical imaging systems, as described in B. J. Lin, Proceedings of the 10th Annual Symposium on Microlithography, pp. 54-79, SPIE vol. 1496, September 1990. However, several problems have to be overcome to facilitate manufacturing applications. One of the key problems is to inspect the phase-shifting mask (PSM) for possible patterning or phase shifting errors. While many phase shifter errors in the lateral dimension can be readily inspected with existing inspection equipments for conventional intensity masks because of the existence of a boundary, phase errors are difficult to inspect. In principle, phase-contrast microscopy as described in M. Born and E. Wolf, "Principles of Optics" Pergamon Press, 3rd Ed., pages 300, 424, (1964) turns phase variations into intensity distributions; thus, rendering quantitative evaluation possible. Most phase shifting masks call for a .pi. phase shift to maximize the imaging improvement but the .pi. phase shift is not the optimum phase shift for phase error detection. By employing the method of this invention, applicant has discovered that a predetermined optimum wavelength for phase error detection can be employed.
Further in accordance with this invention, the wavelength does need not need to be changed, but an optimum inspection phase shifting angle can be used for a given phase shifting angle to be inspected.